U.S. patent number 4,714,632 [Application Number 06/807,890] was granted by the patent office on 1987-12-22 for method of producing silicon diffusion coatings on metal articles.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Alejandro L. Cabrera, John F. Kirner, Robert A. Miller, Ronald Pierantozzi.
United States Patent |
4,714,632 |
Cabrera , et al. |
December 22, 1987 |
Method of producing silicon diffusion coatings on metal
articles
Abstract
A silicon diffusion coating is formed in the surface of a metal
article by exposing the metal article to a reducing atmosphere
followed by treatment in an atmosphere of 1 ppm to 100% by volume
silane, balance hydrogen or hydrogen inert gas mixture. Hydrogen
with a controlled dew point is utilized as a surface preparation
agent and diluent for the silane.
Inventors: |
Cabrera; Alejandro L.
(Fogelsville, PA), Kirner; John F. (Allentown, PA),
Miller; Robert A. (Allentown, PA), Pierantozzi; Ronald
(Orefield, PA) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
Family
ID: |
25197373 |
Appl.
No.: |
06/807,890 |
Filed: |
December 11, 1985 |
Current U.S.
Class: |
427/255.26;
427/343; 427/255.4; 427/318; 427/255.18; 427/255.37 |
Current CPC
Class: |
C23C
10/02 (20130101); C23C 10/60 (20130101); C23C
10/08 (20130101) |
Current International
Class: |
C23C
10/60 (20060101); C23C 10/02 (20060101); C23C
10/08 (20060101); C23C 10/00 (20060101); C23C
016/24 () |
Field of
Search: |
;427/248.1,255.1,318,255.4,343,344,255.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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302305 |
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Dec 1917 |
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DE2 |
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2745812 |
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Apr 1979 |
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DE |
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774656 |
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Dec 1934 |
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FR |
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1530337 |
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Oct 1978 |
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GB |
|
2107360 |
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Apr 1983 |
|
GB |
|
668977 |
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Jun 1979 |
|
SU |
|
Other References
Pons, Galerie, & Caillet, Materials Chemistry and Physics 8
(1983) 153-161. .
Dubois & Nuzzo, Reactivity of Intermetallic Thin Films by
Surface Mediated Decomposition of Main Group Organometallic
Compounds, J. Vac. Sci. Technol. A2(2) Apr.-Jun. 1984, 441-445.
.
Galerie & Caillet, Protection of Iron Against Corrosion by
Surface Siliconization, Materials Chemistry, vol. 5, No. 2, pp.
147-164 (1980). .
Rebuffat, Galerie, Caillet & Besson; La Protection du Auvre par
Siliciuration Superficielle; Materials Chemistry 7 (1982) 517-536.
.
Bertsch & Pretorius; Deactivation of Metal Surfaces for
Capillary Columns for GC by Deposition of Silicon, Journal of HRC
& CC, 493, 499 & 500, 1982. .
Greenberg & Bauer, The Role of Silicon in Corrosion-Resistant
High Temperature Coatings; Metallurgical and Protective Coatings;
pp. 3-20, Thin Solid Films, vol. 95, 1982. .
Wahl and Schmaderrer; The Use of Silicon-Enriched Layers as a
Protection Against Carburization in High Temperature Gas-Cooled
Reactors; Preparation and Characterization (1982), Thin Solid Films
94; 257-268. .
Wahl and Furst; Preparation and Investigation of Layers Enriched in
Silicon by Chemical Vapor Deposition, pp. 333-351. .
Tuler and Schieber; Silicon-Containing Coatings Produced by a
Chemical Vapour Deposition Method on Nickel-Based Superalloys, The
Solid Films 73 (1980) 379-384. .
Nicoll & Hildebrandt & Wahl; The Properties of Chemical
Vapour-Deposited Silicon Base Coating for Gas Turbine Blading, Thin
Solid Films 64 (1979) 321-326. .
P. C. Felix, Coating Requirements for Industrial Gas Turbines, pp.
199-212. .
Hildebrandt, Wahl and Nicoll; Phase Stability of High Temperature
Coatings on NiCr-base Alloys, pp. 213-232. .
Nicoll, Wahl & Hildebrandt; Ductile-Brittle Transition of High
Temperature Coatings for Turbine Blades, pp. 233-252. .
Wahl & Furst; Preparation and Investigation of Silicon-Enriched
Layers on Metals, pp. 529-541. .
Singheiser & Wahl; Protection of Nickel-Based Alloys Against
High Temperature Carburization, pp. 286-292. .
Van Zolingen et al, "Growth Conditions and Properties of Evaporated
Semicrystalline Silicon Layers", Thin Solid Films, vol. 58, pp.
89-93, 1979. .
Gdula et al, "Haze Elimination in Thick Polycrystalline Silicon
Films", IBM Tech. Discl. Bulletin, vol. 21, No. 5, Oct.
1978..
|
Primary Examiner: Childs; Sadie L.
Attorney, Agent or Firm: Simmons; James C. Marsh; William
F.
Claims
Having thus described our invention what is desired to be secured
by Letters Patent of the United States as set forth in the appended
claims:
1. A method of forming a silicon diffusion coating on the surface
of a metal said metal subject formation of a surface oxide that can
be reduced by a furnace treatment under controlled atmosphere, the
steps comprising:
(a) pretreating said metal by heating said metal under conditions
of, temperature less than 1200.degree. C. under a controlled
atmosphere reducing to elemental constituents of said metal to
reduce or prevent formation of a barrier coating on exposed
surfaces of said metal; and
(b) treating said metal under conditions where said metal article
can be maintained at a temperature of less than 1000.degree. C.
under a controlled atmosphere consisting of silane at least 1 part
per million by volume, balance hydrogen or hydrogen and inert gas
mixture wherein said atmosphere contains silane to oxygen in a
molar ratio greater than 2.5 and oxygen to hydrogen in a molar
ratio less than 2.times.10.sup.-4 whereby silicon is diffused into
the surface of said metal article.
2. A process according to claim 1 wherein following said treating
steps said metal is exposed to an atmosphere containing an oxygen
donor whereby at least a portion of said diffused silicon layer is
preferentially oxidized to form a protective coating of silicon
oxides.
3. A process according to claim 2 wherein said oxygen donor is
selected from the group consisting of water vapor and hydrogen;
hydrogen, nitrogen and water vapor; and hydrogen and nitrous
oxide.
4. A process according to claim 2 wherein said atmosphere
containing an oxygen donor is reducing to components of the metal
at the treating temperature.
5. A process according to claim 1 wherein said pretreatment step is
conducted under an atmosphere selected from the group consisting of
hydrogen and hydrogen and inert gas where the molar ratio of oxygen
to hydrogen is less than 2.times.10.sup.-4.
6. A process according to claim 1 wherein the treating step is
carried out in an atmosphere consisting of 1 ppm to 5 per cent by
volume silane, balance hydrogen or hydrogen inert gas mixture.
7. A process according to claim 1 wherein the treatment step is
carried out under an atmosphere containing 500 ppm to 5 per cent by
volume silane balance hydrogen.
8. A process according to claim 1 wherein said process is carried
out in a single furnace in stepwise fashion under an atmosphere
consisting essentially of hydrogen controlled as to, water vapor
content in said pretreating step and hydrogen diluted with silane
and controlled as to water vapor in said treating step.
9. A process according to claim 1 where said metal is maintained at
a temperature of between 500.degree. C. and 1000.degree. C. in both
said pretreating and treating steps.
10. A process according to claim 1 wherein said pretreating and
said treating atmospheres are hydrogen based wherein said hydrogen
has a dew point of -60.degree. C. or below.
11. A process according to claim 1 wherein said metal is a ferrous
metal.
12. A process according to claim 1 wherein said metal is used in a
high temperature oxidizing environment.
13. A process according to claim 2 wherein said metal is a ferrous
metal.
14. A process according to claim 2 wherein said metal is used in a
high temperature oxidizing environment.
15. A method of protecting a metal said metal subject to formation
of a surface oxide that can be reduced by a furnace treatment under
controlled atmosphere by forming a silicon diffusion coating on the
exposed surface of said metal the steps comprising
(a) pretreating said metal by heating in a furnace maintained at a
temperature of at least 400.degree. C. under a furnace atmosphere
reducing to elemental constituents of said metal to reduced or
prevent formation of a barrier film on exposed surfaces of said
metal;
(b) treating said metal in a furnace maintained at a temperature of
at least 400.degree. C. under a furnace atmosphere consisting of
silane at least 500 parts per million by volume balance hydrogen or
hydrogen and inert gas mixture wherein said atmosphere contains
silane to oxygen in a molar ratio greater than 2.5 and oxygen to
hydrogen in a molar ratio less than 2.times.10.sup.-4 whereby
silicon is diffused into the surface of said metal.
16. A process according to claim 15 wherein following said
treatment under silane said article is exposed to an atmosphere
containing an oxygen donor whereby at least a portion of said
diffused silicon layer is preferentially oxidized to form a
protective coating of silicon oxides.
17. A process according to claim 16 wherein said oxygen donor is
selected from the group consiting of water vapor and hydrogen;
hydrogen, nitrogen, and water vapor; and hydrogen and nitrous
oxide.
18. A process according to claim 16 wherein said atmosphere
containing an oxygen donor is reducing to components of the metal
at treating temperature.
19. A process according to claim 15 wherein said pretreatment step
is conducted under an atmosphere of hydrogen where the molar ratio
of oxygen to hydrogen is less than 2.times.10.sup.-4.
20. A process according to claim 15 wherein the treating step is
carried out in an atmosphere consisting of 1 ppm to 5 per cent by
volume silane, balance hydrogen or a hydrogen inert gas
mixture.
21. A process according to claim 15 wherein the treatment step is
carried out under an atmosphere containing 500 ppm to 5 per cent by
volume silane balance hydrogen.
22. A process according to claim 15 wherein said process is carried
out in a single furnace in stepwise fashion under an atmosphere
consisting essentially of hydrogen controlled as to, water vapor
content in said pretreating step and hydrogen diluted with silane
and controlled as to water vapor in said treating step.
23. A process according to claim 15 where said furnace is
maintained at a temperature of between 500.degree. C. and
1000.degree. C. in both said pretreating and treating steps.
24. A process according to claim 15 wherein said pretreating and
said treating atmospheres are hydrogen based wherein said hydrogen
has a dew point of -60.degree. C. or below.
25. A process according to claim 15 wherein said metal is a ferrous
metal.
26. A process according to claim 16 wherein said metal is a ferrous
metal.
27. A method of protecting a metal article said metal article
subject to formation of a surface oxide that can be reduced by a
furnace treatment under controlled atmosphere comprising the steps
of:
(a) pretreating said metal article by heating said metal under
conditions of, temperature less than 1200.degree. C. under a
controlled atmosphere reducing to elemental constituents of said
metal to reduce or prevent formation of a barrier film on exposed
surfaces of said metal;
(b) treating said article to form a silicon diffusion coating on
exposed surfaces of said article; and
(c) exposing said article to an oxidation treatment under an
atmosphere containing an oxygen donor whereby at least a portion of
said diffused silicon layer is preferentially oxidized to form a
protective coating of silicon oxides.
28. A process according to claim 27 wherein said pretreatment step
is conducted under an atmosphere of hydrogen where the molar ratio
of oxygen to hydrogen is less than 2.times.10.sup.-4.
29. A process according to claim 27 wherein the silicon diffusion
coating is formed by heating said metal article in an atmosphere
selected from the group consisting of 1 ppm to 5 per cent by volume
silane and 1 ppm to 5 per cent by volume volatile silicon compound,
balance hydrogen or a hydrogen-inert gas mixture.
30. A process according to claim 27 wherein the silicon diffusion
coating is formed by heating said metal article under an atmosphere
containing 500 ppm to 5 per cent by volume silane balance
hydrogen.
31. A process according to claim 27 wherein said oxygen donor is
selected from the group consisting of water vapor and hydrogen;
hydrogen, nitrogen and water vapor; and hydrogen and nitrous
oxide.
32. A process according to claim 27 wherein said process is carried
out in a single furnace in stepwise fashion under an atmosphere
consisting essentially of hydrogen controlled as to, water vapor
content in said pretreating step and hydrogen diluted with silane
and controlled as to water vapor in said treating step.
33. A process according to claim 27 where said metal is heated to a
temperature of between 500.degree. C. and 1000.degree. C. in both
said pretreating and treating steps.
34. A process according to claim 27 wherein said pretreating and
said treating atmospheres are hydrogen based wherein said hydrogen
has a dew point of -60.degree. C. or below.
35. A process according to claim 27 wherein said metal is a shaped
article used in an ethane cracking environment.
36. A process according to claim 27 wherein said metal is a ferrous
metal.
Description
FIELD OF THE INVENTION
The present invention pertains to the formation of diffusion
coatings in metal surfaces and, in particular, to the formation of
silicon diffusion coatings.
BACKGROUND OF THE PRIOR ART
ln the prior art, it is known that objects which are to be exposed
to reactive atmospheres at high temperatures may be rendered
relatively inert, as compared to the base material, by deposition
of a coating of metallic silicon or silicon oxide on the surface of
the metallic article exposed to the reactive atmosphere and/or high
temperature. In view of the fact that silicon dioxide has a high
melting point, is unreactive toward many common atmosphere systems
and has little catalytic activity, provision of such coatings is
highly desirable. The fact that silicon dioxide has little
catalytic activity has great value in such applications as
equipment for steam cracking of hydrocarbons to produce ethylene.
Secondary reactions which might result in the deposition of carbon
on heat exchanger tubes are minimized with a silicon oxide coating
on the exposed metallic surfaces in such reactors.
A number of processes are known and available for producing a
siliconized surface on a metal, either to produce a silicon-rich or
a silica coating. These methods are:
1. Molten metal or salt baths;
2. Pack cementation which transfers silicon to the metal by
generating a volatile silicon compound in-situ by reaction between
pack solids and a gas;
3. Slurry/sinter, by which a slurry of silicon-containing powder is
applied to a metal, dried and sintered to produce a silicon
coating. In this category, silica coatings are produced by
deposition of silica solids such as sols or sol gel and
sintering.
4. Chemical vapor deposition of silicon via a gaseous or vaporized
silicon compound;
5. Chemical vapor deposition of silica via gaseous silicon and
oxygen sources;
6. Thermal spray of melted, atomized silicon-containing material on
a metal substrate;
7. Ion implantation of silicon;
8. Physical vapor deposition of silicon or silicon oxide.
Chemical vapor deposition of silicon is one of the most desirable
processes for a number of reasons, including such factors as
uniform coating of the substrate, relatively low application
temperatures and the option of forming a silicon diffusion layer,
minimum cleaning of parts after treatment, no high-vacuum
requirement and the fact that the parts are amenable to continuous
processing, ease of surface cleaning and post treatment. In
particular, silane (SiH.sub.4) is an attractive source of silicon
because it is a gas containing only hydrogen and silicon thus
avoiding problems caused by other gaseous or gasified silicon
species such as the corrosion of process equipment or
volatilization of the substrate by halide and other reactions that
prevent formation of a diffusion coating such as carbon deposition
and formation of silicon dioxide.
With processes involving the reaction at the surface of the object
being coated, with a silicon halide such as SiCl.sub.4, Si.sub.2
Cl.sub.6, etc., and hydrogen, the overall reaction results in the
formation of metallic silicon and hydrogen chloride. Silicon
applied in this manner at temperatures greater than 1,000.degree.
C. (1832.degree. F.) tends to diffuse into the substrate metal to
form solid solutions and intermetallic compounds. These diffused
coatings are especially desirable because there is no abrupt
discontinuity in either composition or mechanical properties
between the underlying substrate and the silicon at the surface.
However, halogen-based processes suffer from a number of drawbacks
centered around the reactivity and corrosivity of hydrogen chloride
and other halogen derivatives. For example, iron chloride, which
may be formed in the reaction, is volatile and loss of material
and/or alteration of the composition of the substrate may be
serious.
Another method of depositing metallic silicon is by the thermal
decomposition of silane (SiH.sub.4) to yield silicon metal and
hydrogen. British Patent No. 1,530,337 and British Patent
Application No. 2,107,360A describe methods of applying protective
coatings to metal, metal with an oxide coating, or to graphite.
Critical surfaces in nuclear reactors are protected from oxidation
by coating with silicon at greater than 477.degree. F. (250.degree.
C.) under dry, nonoxidizing conditions followed by oxidizing the
coating at a similar temperature, but under conditions such that
silicon oxidizes faster than the substrate. For example, the
patentees point out in the '337 patent that the 9% chromium steel
was first dried in argon containing 2% hydrogen by heating to
approximately 842.degree. F. (450.degree. C.) until the water vapor
concentration in the effluent was less than 50 ppm followed by an
addition of silane to the gas stream wherein the chromium steel in
the form of tubes was treated for 24 hours at temperatures between
909.degree. and 980.degree. F. (480.degree. C. to 527.degree. C.).
When treated for 6 days with a mixture containing 100 ppm of water
vapor, the tubes exhibited a rate of weight gain per unit area less
than 2% that of untreated tubes when exposed to carbon dioxide at
1035.degree. F. (556.degree. C.) for up to 4.000 hours. These are
overlay coatings in contrast to the diffusion coatings prepared
using silicon halide described above. For example, in patent
application '360A, the applicants point out the importance of
limiting the interdiffusion of Si with compounds of the substrate.
These overlay coatings require long deposition times for their
preparation. It is possible to form Si diffusion coatings using
SiH.sub.4 but this requires higher temperatures. French workers
produced diffusion coatings (solid solutions and metal silicides)
utilizing silane under static conditions at elevated temperatures.
[A. Abba, A. Galerie, and M. Caillet, Materials Chemistry, Vol 5,
147-164 (1980); H. Pons, A. Galerie, and M. Caillet, Materials
Chemistry and Physics, Vol. 8, 153 (1983 ).] For iron and nickel,
these temperatures were as high as 1100.degree. C. (2012.degree.
F.). Others have produced metal silicides using silane on nickel
using sputter-cleaned metal surfaces under high vacuum conditions.
[L. H. Dubois and R. G. Nuzzo. J. Vac. Sci and Technol., A2(2),
441-445 (1984).]
SUMMARY OF THE INVENTION
The present invention provides a process for producing a silicon
diffusion coating on a metal surface by reaction of silane and/or
silanehydrogen mixtures with the metal surface at temperatures
below 1,000.degree. C. (1832.degree. F.) preferably 400.degree. C.
to 1,000.degree. C. The process includes a pretreatment step under
a reducing atmosphere, preferably hydrogen, which is controlled as
to the quantity of oxygen atoms present in the gas to make sure
that the substrate is devoid of any barrier oxide coatings. In the
case of pure hydrogen contaminated by water vapor, control can be
effected by control of the dew point of the hydrogen. After the
pretreatment, exposure to the silane, preferably diluted in
hydrogen, provides the desired silicon diffusion coating. A third
but optional step includes oxidation of the diffused silicon to
provide a coating layer or film of oxides of silicon on the exposed
surface of the treated article. The process differs from the prior
art by utilizing lower temperatures to obtain diffusion coatings
and achieves high deposition rates at these lower temperatures.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1a is a plot of percent atomic concentration (A.C. %) of the
critical elements determined by Auger Electron Spectroscopy (AES)
against sputter time of a sample treated according to the present
invention wherein the water vapor of the atmosphere was maintained
at a maximum of 75 ppm during the silicon deposition step at
500.degree. C.
FIG. 1b is a plot similar to FIG. 1a wherein the water content was
controlled to a maximum of 100 ppm during the silicon deposition
step at 500.degree. C.
FIG. 2a is a plot similar to FIG. 1a of a sample treated according
to the present invention wherein the water vapor was maintained at
150 ppm during the silicon deposition step at 600.degree. C.
FIG. 2b is a plot similar to FIG. 2a wherein the water vapor
content was maintained at 200 ppm during the silicon deposition
step at 600.degree. C.
FIG. 3 is a plot of silane to water vapor ratio versus temperature
showing treatments wherein either silicon diffusion coatings
according to the present invention or silicon overlay coatings can
be produced.
FIG. 4 is a plot of per cent composition of critical elements,
determined by AES, versus sputtering time for a sample treated
according to the prior art using the same alloy sample as in FIG.
1a and FIG. 1b.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is a process for siliconizing metallic
surfaces by reaction of silane, either alone or diluted with
hydrogen and/or hydrogen and an inert gas at temperatures below
1,200.degree. C. (2192.degree. F. to provide controlled silicon
diffusion coatings in the metallic surface. The invention provides
a process for protecting metal surfaces with the diffusion coating
containing metal silicides and/or metalsilicon solid solutions as
significant portions of the total coating. A diffusion coating as
opposed to an overlay coating is achieved by treatment conditions
under which the surface is clean; i.e., there is no surface film
which might act as a diffusion barrier to prevent migration of
silicon into the metal being treated or migration of the elements
of the metal by habit to the surface or which might act as a
passive film to prevent surface catalysis of the silane (SiH.sub.4)
decomposition. According to the present invention a clean surface
can be achieved by maintaining conditions during pretreatment such
that the atmosphere is reducing to all components of the alloy that
will react with oxygen.
The present invention comprises two primary steps with an optional
third step. The first step of the invention includes a pretreatment
wherein the metal article to be treated is exposed at an elevated
temperature (preferably 400.degree. to 1,200.degree. C.) under an
atmosphere that is controlled to reduce or prevent formation of any
oxide film which may act as a barrier coating. While numerous
reducing atmospheres can be used, the preferred atmosphere is
hydrogen which contains only water vapor as a contaminant at levels
above 1 ppm. In this case the water vapor content (dew point) of
the hydrogen is the control parameter. For example, in the
treatment of low alloy steel the water vapor to hydrogen (H.sub.2
O/H.sub.2) molar ratio is maintained at a level that is less than
5.times.10.sup.-4.
The second step comprises exposing the pretreated article to
silane, preferably in a hydrogen carrier gas or in a hydrogen-inert
gas mixture under reducing conditions. In the preferred form of the
invention, the silane is present in an amount from 1 ppm to 100% by
volume, balance hydrogen However, it has been found that silane
present in an amount of 500 ppm to about 5% by volume, balance
hydrogen is very effective. Under these conditions, it has been
found that if the molar oxygen content of the atmosphere is closely
controlled during the treatment step, an effective diffusion
coating is produced. In considering the molar oxygen content of the
atmosphere all sources of oxygen (e.g. water vapor, gaseous oxygen,
carbon dioxide or other oxygen donor) must be taken into account.
For example, at 500.degree. C. according to the present invention,
the molar ratio of silane to oxygen (by this is meant the number of
gram atoms of oxygen) (SiH.sub.4 /O) should be greater than 5 and
the molar ratio of oxygen to hydrogen (O/H.sub.2) should be less
than 1.times.10.sub.- 4 for low alloy steel.
An optional third or post-treatment step comprises exposing the
sample, treated according to the two steps set out above, to
oxidation potential conditions such that oxidation of silicon is
favored over oxidation of the substrate by use of a water
vapor-hydrogen, hydrogen-nitrogen-water vapor or hydrogen-nitrous
oxide atmosphere wherein the molar ratio of oxygen to hydrogen
ratio is controlled, to produce a silicon dioxide coating, film or
layer over the silicon diffusion coating.
According to the present invention, the process is applicable to
all substrates which are amenable to the diffusion of silicon such
as ferrous alloys, non-ferrous alloys and pure metals.
A large number of tests according to the present invention were
conducted and are set out in the following examples.
EXAMPLE 1
Samples of pure iron with approximate dimensions of
0.3.times.0.4.times.0.004" were mounted on the manipulator of a
deposition/surface analysis system. Samples were spot-welded to two
tungsten wires and heated by a high current AC power supply. The
temperature of the sample was monitored by a chromel-alumel
thermocouple which was spot-welded to one face of the sample.
The samples were pretreated in pure H.sub.2 at a dew
point=-60.degree. C. (P.sub.H2O /P.sub.H2
.times.1.times.10.sup.-5), at a flow=1100 standard cubic
centimeters (scc)/min and heated at 800.degree. C. for 60 min.
The SiH.sub.4 /H.sub.2 treatment was performed without interrupting
the H.sub.2 flow. Premixed SiH.sub.4 /H.sub.2 was added to the
H.sub.2 flow until a mixture (by volume) of 0.1% SiH.sub.4 in
H.sub.2 was obtained. The samples were then heated at a temperature
between 500.degree.-700.degree. C. for a time interval between 4-15
min, at a total flow=1320 scc/min.
After the treatments were completed. the samples were analayzed by
Auger electron Spectroscopy (AES) and the surface elemental
compositions are listed in Table 1 below. All the samples are
covered with a thin film of SiO.sub.2 of about 70 .ANG. which
presumably was formed when the samples were exposed to oxygen
contaminants prior to the surface analysis.
The samples were inspected with X-ray fluorescence (XRF) to
determine the elemental bulk composition of deeper layers since the
depth of penetration of this technique is about 3 .mu.m. Elemental
concentrations were calculated from XRF intensities using the
respective X-ray cross sections for normalization, and they are
also displayed in Table 1. The samples were also characterized by
X-ray diffraction (XRD) to determine the phases present and it was
found that the siliconized surface is composed of two phases, FeSi
and Fe.sub.3 Si. The predominant phase at 600.degree. C. is
Fe.sub.3 Si while at 700.degree. C. it is FeSi. The analyses are
summarized in Table 1.
TABLE 1
__________________________________________________________________________
Siliconized Fe Samples Bulk Surface Composition Composition
Treatment in SiH.sub.4 /H.sub.2 (3 .mu.m) (10.ANG.) Phases Present*
Sample No. Temp .degree.C.) t (min) Si % Fe % Si % Fe % .alpha.-Fe
Fe.sub.3 Si FeSi
__________________________________________________________________________
1 -- -- 0.1 99.9 100 S -- -- 2 500 4 0.2 99.8 10.6 89.4 S W -- 3
500 8 0.5 99.5 16.6 78.0 S W W 4 500 15 0.3 99.7 10.1 80.9 S W W 5
600 4 27.0 73.0 42.1 48.7 W S M 6 600 8 28.9 71.1 34.4 54.3 -- S M
7 600 15 22.6 77.4 45.1 47.2 W S S 8 700 4 28.1 71.9 50.8 40.6 W M
S 9 700 8 30.0 70.0 68.4 22.2 W M S 10 700 15 39.9 60.1 91.0 0.0 W
M S
__________________________________________________________________________
*S strong diffraction pattern intensity M moderate intensity W weak
intensity
According to Example 1, the tests demonstrate the formation of iron
sillicide diffusion coatings on a pureiron substrate according to
present invention.
EXAMPLE 2
Samples of AISI type 302 steel with approximate dimensions of
0.3.times.0.4.times.0.002" were prepared, mounted, and treated as
in Example 1. A typical analysis by Atomic Absoprtion spectroscopy
(AAS) of the as-received material yielded a nominal composition 7%
Ni, 18% Cr and 73% Fe.
The sample was heated at 700.degree. C. for 15 min. in atmosphere
(by volume) of 0.1% SiH.sub.4 /H.sub.2 at a total flow=1,320
scc/min. After the treatment was completed, the surface was
analyzed by Auger Electron Spectroscopy (AES) without removing the
sample from the system thus minimizing atmospheric contamination.
The surface composition is set out in Table 2, after treatment and
after mild Argon ion (Ar.sup.+) sputtering which probe the depth of
the coating. The surface is enriched with Nickel (Ni) after the
SiH.sub.4 /H.sub.2 treatment and as determined by X-ray
Photoelectlron Spectroscopy (XPS) the Ni is in the form of Ni
silicide.
TABLE 2
__________________________________________________________________________
Analysis of Siliconized 302 SS AES Atomic % After After Ar.sup.+
XPS Analysis SiH.sub.4 /H.sub.2 Sputtering = Elements Binding
Binding Element Treatment 160 A Detected Energy (eV) References
Energy (eV)
__________________________________________________________________________
Si* 103.6 SiO.sub.2 103.4 Si 31.5 34.4 Si 99.7 Ni.sub.2 Si 100.0 C
14.7 -- Fe 706.8 Fe.degree. 706.8 O 30.7 2.3 Cr 574.1 Cr.degree.
574.1 Cr -- 12.9 Ni 853.1 Ni.sub.2 Si 853.1 Fe 10.7 42.5 O 532.7
SiO.sub.2 533.09 Ni 12.5 7.9 C 284.8 contamination 284.6
__________________________________________________________________________
*Two peaks corresponding to Si are present; nevertheless, the peak
identified as SiO.sub.2 is weak indicating that it comes from
residual oxide. The C and O signals are also very weak.
The foregoing tests demopnstrate the formation of a nickel silicide
diffusion coating on an AISI type 302 stainless steel by the method
of the present invention.
EXAMPLE 3
A sample of 1".times.1/2".times.0.004" AISI type 310 stainless
steel foil was suspended using a quartz wire from a microbalance
inside a quartz tube positioned in a tube furnace. The sample was
treated in flowing dry H.sub.2 (D.P.<-60.degree. C.; H.sub.2
O/H.sub.2 <1.times.10.sup.-5) at 800.degree. C. for 30 min.,
then cooled to 500.degree. C. and treated in flowing dry 0.1%
SiH.sub.4 /H.sub.2 by volume (D.P.<-60.degree. C.; H.sub.2
O/H.sub.2 <1.times.10.sup.-5) for a time (100 min.) long enough
to deposit 0.5 mg Si. Surface analyses showed that the top 90A was
composed primarily of SiO.sub.2 and Ni silicide. The oxide was
presumably formed on exposure of the sample to air during
transport. XPS analysis after removal of the oxide film is set
forth in Table 3. Ni silicide is present on the surface of the
sample as was found in Example 2. An AES depth profile using Ar ion
sputtering showed that the surface layer contained (1) 600 .ANG. of
Ni silicide, (2) 3000 .ANG. region of a mixed Ni/Fe silicide with
gradually decreasing Ni/Fe ratio, and (3) a region of about 3000
.ANG. which is rich in Cr relative to its concentration in the bulk
alloy and depleted in Fe and Ni.
TABLE 3 ______________________________________ XPS Results Conc.
B.E. Ref. Ref. Element rel. at. % (eV) B.E. cpd
______________________________________ 1 Si (2p) 48.5 99.4 100.0
Si, Ni.sub.2 Si 2 Fe (2p) 7.3 706.8 706.8 Fe 3 Ni (2p) 44.1 853.2
853.1 Ni.sub.2 Si ______________________________________
In summary, the results of Examples 2 and 3 show that for austentic
stainless steel at 500.degree. C. to 700.degree. C., Ni and Fe have
diffused to the surface to form a metal silicide layer, with Ni
diffusion apparently being slightly faster than Fe, and have left
behind a region depleted of these elements and rich in Cr.
EXAMPLE 4
Samples of 1".times.1/4".times.1/16" coupons of alloy A182F9 (9%
Cr/1% Mo/Fe) obtained from Metal Samples Co., were cleaned in an
acetone sonic bath. The samples were then treated in a Cahn 2000
microbalance inside a quartz tube heated with a tube furnace. Gas
flowed up the tube and exited at a sidearm. The following
procedures were used for the treatment:
(1) Treat samples at 800.degree. C. for 30 min. in flowing dry
H.sub.2 (D.P.<-60.degree. C., H.sub.2 O/H.sub.2
<1.times.10.sup.-5).
(2) Lower temperature to treatment temperature and switch to
H.sub.2 flow with desired dew point.
(3) Admit 0.5% SiH.sub.4 /H.sub.2 mixture (by volume) to give a
total flow of H.sub.2 /SiH.sub.4= 1220 cc/min. (15 min. at
600.degree. C., 2.5 hr. at 500.degree. C.).
(4) Turn off SiH.sub.4, cool rapidly in H.sub.2.
(5) Determine diffusion vs. overlay coating by AES depth
profiling.
Table 4 summarizes the results of the samples treated as set out
above at 500.degree. C. H.sub.2 O levels of 75 ppm (SiH.sub.4
/H.sub.2 O)=6.7) and lower result in diffusion coatings according
to the present invention whereas H.sub.2 O levels of 100 ppm
(SiH.sub.4 /H.sub.2 O=5) and higher will result in overlay
coatings. FIG. 1a and FIG. 1b compare AES depth profiles for the
diffusion coating at 75 ppm H.sub.2 O to the overlay coating at 100
ppm H.sub.2 O. The sample surface in FIG. 1a was sputtered at a
rate of 15 .ANG./min for six minutes and then at a rate of 150
.ANG./min for five minutes. The sample surface of FIG. 1b was
sputtered at a rate of 10 .ANG./min for twenty minutes and then at
a rate of 130 .ANG./min for 28 minutes.
TABLE 4 ______________________________________ Run H.sub.2 O
SiH.sub.4 SiH.sub.4 / Temp. Number (ppm) (ppm) H.sub.2 O
(.degree.C.) Coating Type ______________________________________ 1
100 500 5.0 500 overlay 2 75 500 6.7 500 diffusion 3 50 500 10.0
500 diffusion 4 20 500 25.0 500 diffusion 5 10 500 50.0 500
diffusion 6 <10 500 >50.0 500 diffusion
______________________________________
Table 5 summarizes the results of the samples treated as set out
above at 600.degree. C.
TABLE 5
__________________________________________________________________________
Run H.sub.2 O SiH.sub.4 Temp. Coating Wt. gain Fe/Si Number (ppm)
(ppm) SiH.sub.4 /H.sub.2 O (.degree.C.) Type mg/cm.sup.2 AES
__________________________________________________________________________
1 200 500 2.5 600 overlay <.02 >19 2 150 500 3.3 600
diffusion 0.03 1.88 3 100 500 5.0 600 diffusion 0.05 1.24 4 50 500
10.0 600 diffusion 0.15 1.24 5 20 500 25.0 600 diffusion 0.20 0.81
6 <10 500 >50 600 diffusion O.43 0.46
__________________________________________________________________________
Increasing the H.sub.2 O level decreases the extent of siliconizing
as evidenced by both the gravimetric uptake (weight gain,
milligrams/sq. centimeter) and by the Fe/Si ratio determined by AES
at the point in the depth profile at which the oxygen content was
insignificant. H.sub.2 O levels of 150 ppm and lower result in
diffusion coatings according to the present invention. H.sub.2 O
levels of 200 ppm and higher will result in overlay coatings. This
is demonstrated by AES depth profiles shown in FIGS. 2a and 2b. The
sample surface of FIG. 2a was sputtered at a rate of 15 .ANG./min
for fourteen minutes and then at a rate of 150 .ANG./min for six
minutes. The sample surface of FIG. 2b was sputtered at a rate of
10 .ANG./min for thirty minutes.
The results at 500.degree. and 600.degree. C. have been combined in
FIG. 3 in a plot which illustrates the relationship between
treatment temperature and the ratio of silane to water vapor in the
atmosphere to effect either diffusion coatings according to the
present invention or overlay coatings.
Example 5 was run to determine results for samples treated
according to the prior art process set out in British Patent No.
1,530,337 and British Patent Application No. 2,107,360A.
EXAMPLE 5
A sample of 1".times.1/4".times.1/16" alloy A182F9 (9% Cr/1% Mo/Fe)
obtained from Metal Samples Co. was suspended using a quartz wire
from a microbalance inside a quartz tube positioned in a tube
furnace. The sample was treated in flowing dry H.sub.2
(D.P.<-60.degree. C.; H.sub.2 O/H.sub.2 <1.times.10.sup.-5)
at 800.degree. C. for 30 min. to remove C, S, and O contaminants,
then cooled to 500.degree. C. The sample was treated according to
the prior art teaching at 500.degree. C. in 2% H.sub.2 /He with a
water vapor content less than 100 ppm (90 ppm; H.sub.2 O/H.sub.2
=4.5.times.10.sup.-3) (for 24 hr). The sample was then treated in
500 ppm SiH.sub.4 /2% H.sub.2 /(He+Ar) with a water vapor content
less than 100 ppm (90 ppm; H.sub.2 O/H.sub.2 =4.5.times.10.sup.-3)
at 500.degree. C. for 24 hr. The sample was cooled rapidly in the
90 ppm H.sub.2 O/2% H.sub.2 /(He+Ar) flow.
The AES depth profile shown in FIG. 4 illustrates that the surface
is covered with an overlay coating containing silicon oxides of
about 0.13 microns thick. The sample surface was sputtered at a
rate of 140 .ANG./min for twenty two minutes. From the results set
out there was no evidence of diffusion of silicon into the surface
of the base metal.
There is an oxide region below the Si-containing overlay coating.
This oxide is about 500 .ANG. thick and was probably formed during
the pretreatment in 2% H.sub.2 /He with 90 ppm H.sub.2 O. The oxide
is enriched in Cr relative to the concentration of Cr in the bulk.
This Cr-rich oxide may be preventing diffusion of Si into the
bulk.
Comparison of Example 5 to Example 4 clearly demonstrates the
difference between the method of the present invention and that of
the prior art for treatment of metals and alloys with
SiH.sub.4.
The treatment according to the present invention under reducing
conditions results in a Si diffusion coating. The treatment
according to the prior art results in a Si-containing overlay
coating of silicon oxides. The rates of deposition are also
significantly enhanced by the method of the present invention. In
example 4 a 1.7 micron (.mu.m) silicon coating was obtained (e.g.
run 6) in 2.5 hours while in example 5 a 0.13 .mu.m coating is
obtained in 24 hours.
Thus considering examples 4 and 5 together, the results demonstrate
the improvement of the present invention over what is believed to
be the closest prior art. The two methods, although they involve
similar treatments with mixtures of the same gases, yield entirely
different and unexpected results. The characteristic of the method
set forth in Example 5 of the prior art yields a highly oxygenated
surface layer and an abrupt discontinuity between the surface layer
and the substrate. This results in what is known as an overlay
coating. The process according to the invention as illustrated by
Example 4, on the other hand, provides a coating which varies
continuously from a superficial oxide coating to a large diffused
silicon layer containing both silicon and iron with a gradual
transition from the high silicon surface down to the base metal.
The coating produced by the process of the invention is a diffusion
coating. A coating of this type will be less subject to thermal or
mechanical shock than the coatings of the prior art. It will also
be self-healing by providing a reservoir of silicon in the base
material. A further advantage of a process according to the present
invention is a relatively greater speed which the coating can be
generated. With a coating according to the present invention a
matter of hours is required whereas according to the prior art
process several days are required to obtain a coating of the same
thickness.
Example 6 demonstrates utility of a type 310 stainless steel with a
selectively oxidized nickel silicide diffusion coating for
inhibiting coke formation when exposed to a simulated ethane
cracking environment.
EXAMPLE 6
A sample of AISI type 310 stainless steel with approximate
dimensions of 0.3.times.0.4.times.0.004" was prepared, mounted, and
treated as in Example 1.
The sample was heated in a 0.1% SiH.sub.4 in H.sub.2 mixture (by
volume) at 700.degree. C. for 15 min. at a total flow=1320
scc/min.
The sample was removed from the surface analysis system and
suspended with a quartz wire from a microbalance inside a quartz
tube positioned in a tube furnace. The sample was treated in dry
H.sub.2 at 1040.degree. C. to reduce the surface. It was then
treated in H.sub.2 /N.sub.2 /H.sub.2 O at a P.sub.H2O /P.sub.H2
=2.1.times.10.sup.-4 to form a SiO.sub.2 surface film.
The sample was cooled to 850.degree. C. and exposed to a simulated
ethane cracking environment (Ethane: 120 cc/min; Nitrogen: 500
cc/min; Ethane H.sub.2 O mole ratio=4) for 1 hr periods. Decoking
was accomplished by turning off the ethane flow for 30 min. No
detectable weight gain was observed (<0.05 .mu.g/sec) for two
coking cycles as compared to weight gains of 0.2-2.6 .mu.g/sec in
the first cycle for control runs.
Example 7 demonstrates that silicon diffusion coatings can be
effectively produced on pure metals (e.g. iron) using the process
of the present invention.
EXAMPLE 7
Samples of 1".times.0.5".times.0.002" foils of pure Fe from Alfa
(99.99% pure), cleaned in an acetone sonic bath and hung from a
micro balance. Samples were then treated in the following
manner:
(1) Treat sample at 800.degree. C. for 1 hr. in flowing dry H.sub.2
(D.P.=-54.7.degree. C.).
(2) Lower temperature to 500.degree. C. and select desired dew
point in the H.sub.2 flow.
(3) Admit 0.5% SiH.sub.4 /H.sub.2 at a flow rate that yielded a
final mixture of 800 ppm in H.sub.2 for 15-30 min (total flow=480
cc/min).
(4) Turn off SiH.sub.4 /H.sub.2 mixture, purge with dry He and cool
down to room temperature.
(5) Analyze surface composition using AES depth profiling to
determine diffusion vs. overlay coating.
Using AES depth profiling, a diffusion coating is observed in a Fe
sample that was siliconized at 500.degree. C. with a mixture of 800
ppm SiH.sub.4 and 25 ppm H.sub.2 O in H.sub.2 (SiH.sub.4 /H.sub.2
O=32 ).
The results set forth in Example 8 demonstrate that silicon
diffusion coatings can be produced for high temperature oxidation
protection of various metal parts.
EXAMPLE 8
A sample of 1.0.times.0.5.times.0.002" carbon steel 1010 (99.2% Fe)
obtained from Teledyne Rodney Metals was suspended using a quartz
wire from a microbalance inside a quartz tube positioned in a tube
furnace. The sample was treated in flowing dry H.sub.2
(D.P.=-60.degree. C.) at 800.degree. C. for 1 hour at a flow of 400
cc/min and then cooled to 600.degree. C. The sample was then
treated in a mixture of 0.12% SiH.sub.4 in H.sub.2 (by volume)
until it gained 2 mg in weight and then cooled rapidly in flowing
H.sub.2 . It was estimated that a Fe.sub.3 Si diffusion coating of
about 3 .mu.m was formed with this treatment.
After this siliconizing step, the sample was kept under flowing He
and heated up to 800.degree. C. The gas flow was then switched to
pure O.sub.2 and the weight increase due to oxidation was monitored
for 1 hour. The sample yielded a linear oxidation rate of 0.23
.mu.g.times.cm.sup.-2.times.min.sup.-1 and the adhesion of the
surface film was good. An untreated sample of carbon steel 1010
yielded an oxidation rate of 2.7.times.10.sup.4
.mu.g.times.cm.sup.-2 .times.min.sup.-1 under identical conditions.
Therefore, there was a reduction of 1.2.times.10.sup.5 times in the
oxidation rate for the siliconized sample.
From the foregoing examples, it is apparent that processes
according to the present invention can be utilized to provide
silicon diffusion in a metal or other substrate. The present
invention is distinguished over the prior art by the fact that the
present invention teaches the use of a pretreatment to remove any
diffusion barriers such as oxide films or carbon impurities on the
surface of the substrate which might inhibit the deposition of the
silicon on the surface and the diffusion of the silicon into the
surface of the substrate. As amply demonstrated above the process
is effected by carefully controlling the water vapor content of the
reducing atmosphere during the pretreatment step and the water
vapor content of the atmosphere and the ratio of silane to water
vapor during the treatment step.
Thus according to the present invention many substrates can be
given a diffusion coating of silicon which coating can subsequently
be oxidized to provide a silicon dioxide coating which will resist
attack under various conditions of use.
* * * * *